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Thanks Builder, I know 'tis the season of mist...' but I'm not too keen on it on the inside of my windows ;o) We do have uPVC double glazing Deggers, which is normally securely closed during darkness hours to keep out the draughts and early morning noise from some rather inconsiderate neighbours. We can lock off the windows to leave a tiny gap so I'll give that a try and see if it helps, may also look at the vents for a more permanent answer. Thanks for your help.